This invention relates to coating fluids for printing plates and a method of making a printing plate. More particularly, this invention relates to a coating fluid used to make printing plates which permit images to be written in response to input digital data and which can be easily regenerated and reused, as well as a method of making a printing plate by using this coating fluid.
Recently, the printing process is increasingly converted to digital form in the general field of printing technology. According to this digital technology, an image or manuscript is prepared on a personal computer or an image is read with a scanner or the like to convert the image data to digital form, and a printing plate is made directly from such digital data. Thus, a saving of labor can be achieved throughout the printing process and highly precise printing can be easily made.
Conventionally, so-called PS plates (presensitized plates) having a hydrophilic non-image area comprising anodized aluminum and a hydrophobic image area formed by curing a photosensitive resin on the surface thereof have been commonly used as printing plates.
When such PS plates are used, a plurality of steps are required to make printing plates, so that plate making consumes much time and involves considerable costs. Consequently, it is difficult to push forward a shortening of the time required for the printing process and a reduction in printing costs. In particular, the use of PS plates for the printing of a small number of copies causes an increase in printing costs. Moreover, PS plates require a development step using a developing solution. Not only this development step requires much labor, but also the disposal of waste liquor from the development step poses an important problem from the viewpoint of the prevention of environmental pollution.
Moreover, a PS plate is generally exposed to light by preparing a film having an original image pierced therethrough and bringing this film into close contact with the plate surface. This constitutes an obstacle to the preparation of a printing plate directly from digital data, i.e. the promotion of digital printing technology. Another problem is that, after the printing of one image is finished, the plate must be replaced for the next printing step and is hence discarded.
Furthermore, other known methods of making a printing plate include, for example, a method in which a laser absorption layer comprising carbon black or the like and a silicone resin layer are successively applied to a PET (polyethylene terephthalate) film, an image is written with a laser beam to generate heat from the laser absorption layer, and the silicone resin layer is burned off by that heat to make a printing plate; and a method in which a lipophilic laser absorption layer is applied to an aluminum plate, a hydrophilic layer is further applied thereto, and the hydrophilic layer is burned off with a laser beam in the same manner as described above to make a printing plate.
In addition, a method of making a printing plate by using a plate comprising a hydrophilic polymer and irradiating it with light according to image data so as to lipophilize the irradiated part has been proposed.
According to these methods, printing plates can be made directly from digital data. However, after the printing of one image is finished, the plate must be replaced by a new one for the next printing step. Accordingly, they are not essentially different in that the printing plates once used are discarded.
Thus, printing plates which permit images to be written with light such as infrared radiation (IR) are known, but most of them cannot be regenerated. Recently, a system which permits image writing with IR and plate regeneration has been proposed. In this system, however, a photocatalyst is not used as a component of a printing plate. Accordingly, as a means for regenerating a plate after completion of printing, it is employed to wash off the image area adhering to the substrate solely by use of cleaning fluids. Consequently, this system cannot be said to be convenient, for example, in that a variety of cleaning fluids must be used in large amounts.
Meanwhile, several inventions relating to regenerable printing plates using a titanium dioxide photocatalyst have been published, and it is set forth in most of them that the titanium dioxide itself can be hydrophobized by the application of heat. However, it is very difficult in theory to hydrophobize a titanium dioxide photocatalyst by the application of heat. The sole instance in which the hydrophobization of a titanium dioxide photocatalyst is considered to be basically possible is the case in which a titanium dioxide photocatalyst having Cr ions injected thereinto is exposed to visible light. However, since a long time is required for the purpose of hydrophilic-hydrophobic conversion, it is practically difficult to apply this method to plate regeneration.
On the other hand, there have also been proposed a technique for making a printing plate in which a TiO2 photocatalyst is made hydrophilic by irradiation with ultraviolet radiation (UV) and at least a part of the photocatalyst is hydrophobized by the application of heat to form a latent image, and such a technique in which light such as IR is used as the heating means.
However, the hydrophilization of a TiO2 photocatalyst by the application of heat has been published by Prof. Fujishima (at the University of Tokyo) and others. Accordingly, it has been basically impossible to carry out image writing or plate regeneration on the basis of the hydrophobization of TiO2 by the application of heat.
In view of the above-described problems, an object of the present invention is to provide a coating fluid for printing plates containing thermoplastic resin particles and, more particularly, a coating fluid for printing plates which is suitable for use in a method for writing images on a regenerable printing plate using a TiO2 photocatalyst with IR that is inert light to the photocatalyst, and a method of making a printing plate by using the coating fluid for printing plates.
A coating fluid for printing plates in accordance with the present invention comprises at least a carrier liquid, thermoplastic resin particles and an IR absorber, and the IR absorber has a decomposition starting temperature higher than the melt starting temperature of the thermoplastic resin particles.
When the coating fluid for printing plates in accordance with the present invention is used, the heat converted by the IR absorber is not absorbed by the absorber itself before the thermoplastic resin particles begin to melt, so that the heat converted by the IR absorber can be efficiently absorbed by the thermoplastic resin particles to melt them rapidly. Consequently, the required output of an image writing apparatus using IR can be reduced to achieve a reduction in the cost of the apparatus and a saving of energy.
The aforesaid IR absorber may be contained in or attached to the thermoplastic resin particles.
Then, it is preferable that the light absorptivity of the composite material consisting of the IR absorber and the thermoplastic resin have a peak at a wavelength of 800 to 850 nm and preferably about 830 nm.
The present invention also relates to a method of making a printing plate by forming a hydrophobic image area in at least a part of a hydrophilic printing plate surface containing a photocatalyst. This method comprising a hydrophobizing agent application step for applying a coating fluid for printing plates, as a hydrophobizing agent, to the plate surface, the coating fluid for printing plates comprising at least a carrier liquid, thermoplastic resin particles and an IR absorber, the IR absorber having a decomposition starting temperature higher than the melt starting temperature of the thermoplastic resin particles; an image writing step for forming a hydrophobic image area by heat-treating at least a part of the plate surface; and a hydrophobizing agent removal step for removing the hydrophobizing agent applied to other areas of the plate surface than the hydrophobic image area.
According to the present method of making a printing plate by using the aforesaid coating fluid for printing plates as a hydrophobizing agent, a hydrophobic image area can be efficiently formed by use of IR light. That is, it is possible to make printing plates having high efficiency in the utilization of IR energy and a high speed of image writing with IR. Moreover, an enhancement in image writing speed makes it possible to shorten the time required for the preparation of plate making and thereby improve the operating efficiency of the printing machine.
The aforesaid photocatalyst preferably comprises titanium dioxide and, in particular, anatase type titanium dioxide.
Moreover, the aforesaid IR absorber may be contained in or attached to the thermoplastic resin particles.
Then, it is preferable that the light absorptivity of the composite material consisting of the IR absorber and the thermoplastic resin have a peak at a wavelength of 800 to 850 nm and preferably about 830 nm.